A basic goal of a wafer fabrication plant is reduction of particulate contamination, and one of the times at which this type of contamination needs to be kept as small as possible is when the wafer is being inspected or examined, using a scanning electron microscope (SEM) or microscope that uses other investigative charged particles such as gallium ions. Since the microscope operates under high vacuum, the evacuation of the microscope of itself reduces contamination by unwanted particles. However, the evacuating system may introduce unwanted particles, and/or the charged particles used by the microscope may also introduce unwanted particles, as is exemplified by the following prior art.
U.S. Pat. No. 6,534,775 to Harrington, et al., whose disclosure is incorporated herein by reference, describes a system and method for inhibiting the transport of particles entrained in a ion beam. The particles within the beam are charged to a polarity in a first region, and a first electric field diverts the charged particles. An electrode downstream from the first electric field provides a potential barrier which repels the charged particles. As a result, the charged particles may be directed away from the direction of beam travel, out of the ion beam, thereby mitigating contamination of a workpiece.
U.S. Pat. No. 6,689,930 to Pang, et al., whose disclosure is incorporated herein by reference, describes a system for eliminating contaminants and residues from inside a vacuum exhaust line connected to a processing chamber. Powder residue and other particulate matter that would otherwise collect in the vacuum line during deposition steps are trapped in a collection chamber and removed through a plasma formed downstream of the reaction chamber.
One of the means of support for the wafer in an SEM or other charged particle microscope is an electrostatic chuck. Electrostatic chucks operate by supporting the wafer mechanically at a number of points beneath the wafer, while using these points to also impart a charge to the wafer. Forces on different portions of the charged wafer may be generated by applying respective electric fields to these portions, using an electrode incorporated in the chuck. The electric fields allow, for example, forces of gravity which may locally bend the wafer to be compensated for.
U.S. Pat. No. 6,451,176 to Vernon, et al., whose disclosure is incorporated herein by reference, describes interception and trapping of charged particulate matter generated by ion beam sputter deposition. An electrostatic particle trap generates electrostatic fields in the vicinity of a substrate on which target material is being deposited. The electrostatic particle trap consists of an array of electrode surfaces, each maintained at an electrostatic potential, having their surfaces parallel or perpendicular to the surface of the substrate.